Giardia lamblia is a major cause of waterborne enteric disease throughout the world. Giardiasis is especially prevalent in children and may cause severe diarrhea, malabsorption, and failure to thrive. Based on extensive data from earlier studies, we now propose to test the following hypotheses: 1) That nonimmune secretory factors in human breast milk inhibit or kill Giardia lamblia under conditions which model the small intestine. 2) That secretory antibodies to G. lamblia act by inhibiting attachment of the trophozoites rather than killing them. The overall goal of this project is to identify factors in human milk which kill G. lamblia or inhibit its attachment in a model of the small intestinal tract of the breast-fed baby. Accordingly, Specific Aim I is to investigate non-immune defense factors in human milk. (A) We will determine whether human milk bile salt stimulated lipase can kill G.lamblia in the presence of a functional human intestinal epithelial monolayer (the T84 cell line). (B) We will use the same system to determine whether human milk lactoferrin (Lf) kills G. lamblia and whether sub-lethal concentrations of Lf inhibit attachment to T84 cells. (C) We will ask whether lysozyme purified from human milk can digest G. lamblia cysts under conditions which mimic their passage through the infant's stomach. Specific Aim II is to investigate the interactions between G. lamblia and specific antibodies in human milk. (A) We will determine whether milk antibodies inhibit trophozoite attachment to T84 cells and whether inhibition is associated with certain antibody isotypes or recognition of specific parasite antigens. (B) We will ask whether the mechanism of inhibition is by blocking specific attachment ligands or by interfering with the function of the adhesive disc or flagella. In Specific Aim III, we will examine the effects of specific intestinal conditions on Lf and milk antibody interactions with trophozoites. In Specific Aim IV, we will determine whether Lf or milk antibodies can passively protect suckling mice from infection with G. lamblia. The work proposed should lead to new insights into the mechanisms of immune and non-immune protection of the infant by human milk and could lead to new strategies to decrease the burden of diarrheal disease in infants.